We shall continue our studies on transformation with purified chromosomes and extend our studies currently in vogue on DNA transformation of mammalian cells. Further improvement in the efficiency of selection and transformation should be useful for the isolation and purification of specific DNA segments from mammalian cells. Reverse phase chromatography for purification of DNA segments has developed to a practical stage and will in turn be exploited for further improving the efficiency of transformation. Procedures for improved selection of mutants have been used to select for HPRTase revertants and will be extended in a "mutant hunt" for cells resistant to various toxic agents. New mutants are needed to pursue studies on homospecific transformation. Methods for controlling nuclease activity have been developed and will be employed to obtain increase levels of uptake of biologically active material. Further studies will be made on the effect of size and structure of DNA on uptake and transport of DNA to the nucleus.